I've just installed an initial implementation of PHG (power-height-gain) circles on the live APRS map. They can be enabled by clicking on the new PHG icon in the top right corner. PHG circles are shown for APRS stations which are transmitting the PHG extension in their comment text.
The first click enables half-sized circles, which are supposed to resemble the reach of an APRS digipeater from a mobile user's point of view (small antenna at low height, lots of fading and multi-path propagation). The second click enables full-sized PHG circles which are calculated according to the APRS 1.0 specification, and might be quite optimistic for mobile users, but more realistic for connectivity between digipeaters and other installations with better antennas. Not surprisingly, the third click disables PHG circles again.
If you'd like to add the PHG extension to your digipeater configuration, please try the NashvilleAPRS PHG Calculator!
Update: There's a better PHG calculator at M0CYP's site which does PHGR - PHG with beacon Rate. aprs.fi doesn't display the rate yet, though.